Photosynthesis in plants makes useful organic compounds out of carbon dioxide through carbon-fixation reactions

The process of photosynthesis in plants involves a series of steps and reactions that use solar energy, water, and carbon dioxide to produce oxygen and organic compounds. Carbon dioxide serves as the source of carbon, and it enters the photosynthetic process in a series of reactions called the carbon-fixation reactions (also known as the dark reactions). These reactions follow the energy-transduction reactions (or light reactions) that convert solar energy into chemical energy in the form of ATP and NADPH, which provide energy to drive the carbon-fixation reactions.

CO2 enters most plants through pores (stomata) in the leaf or stem surface. In photosynthetic algae and cyanobacteria, CO2 is taken up from the surrounding water. Once in a photosynthetic cell, CO2 is “fixed” (covalently bonded) to an organic molecule with the help of the enzyme. In many plant species, this initial reaction is catalyzed by the enzyme Rubisco—the world’s most abundant enzyme.

In a cyclic series of reactions called the Calvin cycle or C3 pathway, the carbon-containing molecule resulting from this first fixation reaction is chemically reduced and converted into various compounds using the energy from ATP and NADPH. The products of the Calvin cycle include a simple sugar that is subsequently converted into carbohydrates like glucose, sucrose, and starch, which serve as important energy sources for the plant. The cycle also regenerates molecules of the initial reactant that more CO2 will bond with in another turn of the cycle. 

Interest in learning from and applying how plants activate and convert CO2 into useful products is particularly high, as CO2 is abundant in the atmosphere but is chemically stable and requires a large amount of energy to convert into compounds that are useful in industrial processes.

For more information on other parts of the photosynthetic process, check out these related strategies:
Pigment molecules absorb and transfer solar energy: Cooke’s koki’o
Catalyst facilitates water-splitting: plants
Photosynthesis converts solar energy into chemical energy: plants


“The Calvin cycle utilises the products of the light reactions of photosynthesis, ATP and NADPH, to fix atmospheric CO2 into carbon skeletons that are used directly for starch and sucrose biosynthesis (Figure 1) (Woodrow and Berry 1988; Geiger and Servaites 1995; Quick and Neuhaus 1997). This cycle comprises 11 different enzymes, catalysing 13 reactions, and is initiated by the enzyme ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco) which catalyses the carboxylation of the CO2 acceptor molecule, ribulose-1,5-bisphosphate (RuBP). The 3-phosphoglycerate (3-PGA) formed by this reaction is then utilised to form the triose phosphates, glyceraldehyde phosphate (G-3-P) and dihydroxyacetone phosphate (DHAP), via two reactions that consume ATP and NADPH. The regenerative phase of the cycle involves a series of reactions that convert triose phosphates into the CO2 acceptor molecule, RuBP. The majority of the triose phosphate produced in the Calvin cycle remains within the cycle to regenerate RuBP. However, carbon compounds produced in this cycle are essential for growth and development of the plant and therefore triose phosphates exit from the cycle and are used to synthesise sucrose and starch.” (Raines 2003:2)
"The chemical activation of CO2, that is, the splitting of its structure in a chemical reaction, is a major challenge in synthetic chemistry because of the very high thermodynamic stability of CO2, which requires an efficient energy source for its activation. However, the fact that biogenic carbon (i.e., biomass) originates from the fixation of CO2 implies that CO2 activation must be one of the oldest reactions in biological systems and have already occurred in prebiotic times.[1], [2] Interestingly, in current photosynthetic systems, this process relies on the formation of a carbamate as the first step of the cycle,[3] which may also have been the case in prebiotic systems, as a number of cyanide-based, nitrogen-rich, conjugated organic molecules, such as nucleic acids, porphyrins, and phthalocyanines, existed before life began." (Goettmann et al. 2007:2717)

Journal article
Metal-Free Activation of CO2 by Mesoporous Graphitic Carbon Nitride

Journal article
The Calvin cycle revisitedRaines CA

Carbon dioxide fixationBrookhaven National LaboratoryFujita E; DuBois DL